bug#35919: recutils package does not install emacs rec-mode

[Jack Hill]

On Sun, 9 Jun 2019, Oleg Pykhalov wrote:

> WDYT about creating a new package recipe in
> ‘gnu/packages/emacs-xyz.scm’, which inherits ‘recutils’ and sets build
> system to ‘emacs-build-system’?  We will probably need to change
> directory after unpack phase in this case, but we will not need to
> maintain Emacs lisp files compilation manually.

I have prepared the following patch implementing this suggestion (thanks 
again, I'm much happier with this). I ended up putting it in databases.scm 
rather than emacs-xyz.scm so that it can live with the recutls definition, 
and not run afoul of the advice by Ludo’ in [0], "The lesson is: when 
inheriting from a package, do it in the module where it’s defined."

[0] https://issues.guix.gnu.org/issue/35154#5

I have also set the inputs and native inputs to the empty lists, because 
as far as I can tell, the emacs mode does not depend on anything outside 
of the emacs-build-system.

Best,
Jack